1. Field of the Invention
The present invention relates to an ink jet recording head, and more particularly, to a so-called gradation-controllable ink jet head in which a plurality of electro-thermal transducers are arranged on respective nozzles for ejecting ink droplets, whereby the amount of the ejected ink droplets can be changed in accordance with image data.
2. Description of the Related Art
Hitherto, there have been two typical systems for constituting an ink jet head: a piezoelectric system utilizing a piezoelectric element, and a bubble jet system in which an electro-thermal transducer is formed on a substrate and nozzles are formed thereon.
A description will be given of modulation of the amount of the ejected ink droplets relating to an object of the present invention described below.
The piezoelectric system can control the amount of the ejected ink droplets within a relatively wide range by modulating a driving waveform of the piezoelectric element, and is suitable for controlling gradation. However, since the piezoelectric element is utilized, the manufacturing process thereof becomes complicated and the system is not so suitable for a high-density arrangement of nozzles.
The bubble jet system has high productivity and excellent high-density arrangement of the nozzles as compared with the above piezoelectric system, and is suitable for producing a high-speed ink jet head at a low cost. It is difficult, however, for the bubble jet system to modulate the amount of the ejected ink droplets. Thus, as a gradation control art in the bubble jet system, a multi-level system has been proposed in which the density of the nozzle arrangement is increased, the amount of the ejected ink droplets is decreased to, for example, about 10 picoliters, and one pixel is represented by many dots.
The multilevel system, however, encounters the following problems. Since the pixel is constituted by fine ink droplets as described above, the number of droplets-driving pulses increases in accordance with the number of dots, thereby shortening the life of the ink jet head. In addition, at the head driving frequency which is the same as that of the conventional head, the recording speed is decreased.
In order to solve the foregoing problems, an ink jet head is proposed in which a plurality of electro-thermal transducers are arranged in one nozzle, and the electro-thermal transducers are actuated as needed, thereby changing the amount of the ejected ink droplets.
The structure of the above conventional ink jet head is shown in FIG. 2. In addition, FIG. 3 shows a cross-sectional configuration of the portion of the electro-thermal transducer taken along the line A-A' of FIG. 2.
Referring to the drawings, a resistive layer 4 formed by a resistor material, such as HfB.sub.2, a wiring layer 5 formed of Al, and a protective layer 6 formed of insulating material, such as SiO.sub.2 are formed on a silicon substrate having a heat storage layer formed thereon. Although a cavitation resistant layer and a protective layer, etc. are further formed thereon, they are omitted from the drawing.
The above conventional ink jet head encounters the following problems.
In the ink jet head in which a plurality of electro-thermal transducers 2 are arranged in one nozzle 1, it is possible to actuate a necessary electro-thermal transducer 2 and to change the volume of an air-bubble in accordance with a necessary amount of ejection in order to change the heat generating area of the electro-thermal transducer 2. The nozzle 1 ejecting the ink droplets is, however, common to each of the electro-thermal transducers 2. Therefore, it is difficult to optimally design the nozzle 1 corresponding to individual amounts of ejection. Particularly, since an ejection port 11 of the tip of the nozzle 1 is fixed, it is almost impossible to optimize the amount of ejection and ejection speed simultaneously.
In order to solve the foregoing problems, the position of the electro-thermal transducers may be changed. This is intended to obtain a necessary amount of ejection and ejection speed by optimizing each of the center points of air bubbles in the electro-thermal transducers.
FIG. 10 shows the relationship between the ejection speed and the amount of ejection when the center of gravity of the electro-thermal transducer is changed. More specifically, FIG. 10 shows the amount of ejection and the ejection speed when a small-area heater and a large-area heater of the electro-thermal transducer are actuated singly and in combination.
It is apparent from FIG. 10 that the amount of ejection and the ejection speed change in response to the area of the electro-thermal transducer, but they are in a direct proportional relationship.
In other words, it is understood that the ejection speed increases when the electro-thermal transducer is located forward of the nozzle (L=50.mu.) as compared with when located rearward of the nozzle (L=100.mu.) in spite of an equivalent amount of ejection.
In addition, as the amount of ejection decreases, the ejection speed decreases. In the most extreme case of decrease, only about 2 to 3 m/s could be obtained. With such properties, it is difficult to maintain reliability.
As described above, the center of gravity of the electro-thermal transducer in relation to the position of the nozzle is a large determinant of the amount of ejection and the ejection speed.
However, in order to satisfy ejection performance required for product specifications, the degree of freedom of design parameters is limited.
For example, in order to achieve a target drive frequency, the entire length of the nozzle should be short to some extent.
In addition, in order to achieve the target amount of ejection, a necessary volume of air bubble should be secured, so that the area of the electro-thermal transducer is determined.
Therefore, even if the position of the electro-thermal transducer is to be changed as described above, it is often impossible to place the electro-thermal transducer in a given nozzle while securing a necessary area of the electro-thermal transducer.
In FIG. 2, for example, two electro-thermal transducers 2 are arranged in one nozzle 1 so as to obtain the ejection amounts of 40 picoliters and 80 picoliters.
In order to achieve an ejection amount of 40 picoliters, an area of about 2000 square microns is required, and an area of about 4000 square microns is required in order to achieve an ejection amount of 80 picoliters, although they are depend on the driving condition of the electro-thermal transducers 2.
To arrange these electro-thermal transducers 2 in the same nozzle 1, the width of the heater is defined due to the restriction in size, so that the length of the heater is also defined.
For example, in order to obtain the above areas, the heaters having a size of 16.mu.125.mu., and a size of 22.mu..times.178.mu. are required, respectively.
When these two electro-thermal transducers 2 are to be arranged by shifting by 100.mu., the entire length of the heater portions thereof becomes 278.mu.. For example, when they are to be arranged in a nozzle having a length of 300.mu., the rear end of the heater portion will protrude from the nozzle and hence, it is impossible to eject properly the ink droplets. Therefore, the amount of shift of the electro-thermal transducers should be reduced.
FIG. 11 illustrates the above-described arrangement of the electro-thermal transducers.
As is apparent from the drawing, the rear end of an air bubble protrudes from a nozzle region at timing when the volume of the air bubble is maximum.